1. Field of the Invention
The present invention relates to a rear-illuminated-type photodiode array that can be used as a receiving section for optical communication or a two-dimensionally arranged optical sensor.
2. Description of the Background Art
The field of optical communication has been achieving widespread adoption of the wavelength division multiplexing (WDM) technology, which enables the transmission and reception of a plurality of optical signals having different wavelengths through a single optical fiber. After the separation of the wavelength-multiplexed optical signals having traveled over a single optical fiber, in order to receive individual optical signals having a different wavelength, a group of independent photodiodes or a photodiode array is used. Because the diameter of the cladding of a single-mode optical fiber is 125 μm, it is desirable that the photodiodes be arranged with a pitch of 125 μm. However, in the case of independent photodiodes, it is difficult to reduce the pitch to such a small value.
In addition to the optical communication use, a photodiode array has a wide range of application. For example, it is used for a two-dimensionally arranged optical sensor and a sensor section of an image pickup tube. When a photodiode array has a small arranging pitch, if the light beam is slanted, the light-entering position can not be aligned precisely with the light-absorbing position when a rear-illuminated-type array is used. In this case, therefore, a front-illuminated-type array is used. Nevertheless, a rear-illuminated-type array can be used in some applications. When the light beam is nearly perpendicular to the light-entering plane, a rear-illuminated-type can also be used.
A photodiode array has a multitude of photodiode units arranged one- or two-dimensionally on one chip. The array has a structure in which a common n-electrode, a common n-type substrate, an absorption layer, a window layer, a plurality of p-regions, a plurality of p-electrodes, and so on are stacked on top of each other in layers.
Because photodiode units having a comparable function are arranged with a small pitch, electrical crosstalk poses a problem. The electrical crosstalk is caused through the following process. First, a lightwave having entered the vicinity of the pn junction of a photodiode produces electron-hole pairs. Then, some holes and electrons flow into a neighboring photodiode unit and produce a photocurrent for the neighboring photodiode unit. Consequently, the electrical crosstalk can be reduced by providing a groove to isolate photodiode units.
The published Japanese patent application Tokukai 2001-144278 has proposed a front-illuminated-type photodiode array provided with grooves between photodiode units. The groove has a depth reaching the vicinity of the substrate to isolate an absorption layer and a pn junction from those of another photodiode unit so that the electrical crosstalk can be reduced. The side wall of the isolation groove is coated with an insulation film, made of SiN or another material, formed by CVD or another proper method. Thus, the pn junction is protected.
Another published Japanese patent application, Tokukai 2001-352094, has also proposed a front-illuminated-type photodiode array provided with isolation grooves to reduce the electrical crosstalk. In this structure, first, the p-region is formed by epitaxial growth for an Si-based photodiode array. Then, the isolation grooves reaching the substrate are formed by etching to isolate photodiode units.
Both of the proposed arrays are front-illuminated-type photodiode arrays. Without regard to the type of photodiode, whether it is the front-illuminated-type or the rear-illuminated-type, a signal lightwave having entered an individual photodiode is absorbed by the individual absorption layer and produces electron-hole pairs. When the electrons and holes cross the pn junction, they produce a photocurrent.
However, not all of an incoming lightwave is absorbed in the absorption layer. A part of it passes through the absorption layer as a leakage lightwave and hits an electrode provided at the other end. The electrode is provided by forming an alloy such as AuZn on an InP crystal and then heating them to bond them mutually with ohmic contact. At this moment, constituents of the electrode and InP are mixed and recrystallization occurs. As a result, a complex crystal boundary is formed between the electrode and the InP crystal. The formed boundary surface is irregular, so that a lightwave is reflected from the boundary surface irregularly. Therefore, the leakage lightwave is irregularly reflected from the electrode at the other end, and part of it enters a neighboring unit photodiode and reaches its pn junction and absorption layer. That is, the reflected lightwave produces a photocurrent in the neighboring photodiode. In other words, a lightwave having entered a photodiode unit causes a noise to a neighboring photodiode unit. This is known as optical crosstalk.
As the spacing between neighboring photodiodes decreases, such optical crosstalk increases accordingly. The optical crosstalk can occur not only between the closest photodiode units but also between the second closest photodiode units and between the third closest photodiode units.